1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method and an image forming program, particularly to an image forming apparatus, an image forming method and an image forming program for forming an image by distribution of two or more types of dots having different densities per unit area.
2. Background of the Invention
A printing apparatus for printing an image by a personal computer or the like is exemplified by a printer using the ink of three colors, magenta (M), cyan (C) and yellow (Y), and a printer using the ink of four colors composed of the aforementioned three colors plus black (K). Such a printer allows an image to be formed by turning on or off the dot of each hue.
In recent years, by contrast, there have appeared many types of printers wherein, in order to get a high-quality output image, inks of different densities within the same hue (e.g. dark colored ink and light color ink) are used or different ink dot diameters are utilized so that a dot having a plurality of gradation levels in one hue is formed.
However, even if the aforementioned dots each having a plurality of gradation levels in one hue can be formed, the image may deteriorate in some cases if these dots are not arranged adequately. For example, to get an output image having a large dot (having a large area, hence high dot density per unit area) and a small dot (having a small area, hence low dot density per unit area), binary halftone processing is applied to each dot; then the pixel values are added up. Thus, although the distribution of small dots alone (FIG. 13(a)) and distribution of large dots alone (FIG. 13(b)) are satisfactory, distribution of the overlapped dots cannot be said to be satisfactory in many cases, with the result that the graininess in the final image considerably deteriorate (FIG. 13(c)).
Solution to this problem has been found in the method wherein a plurality of types of dots having the different densities per unit area are arranged in the form associated with one another. For example, such a method is disclosed in Patent Documents 1 and 2.
In Patent Document 1, a lump formed by a collection of small dots is replaced by a medium dot and the lump formed by a collection of medium dots is replaced by a large dot.
In Patent Document 2, 100% or more of light dots (lower density per unit area) are introduced. When there is a reduction in the difference in density from the dark dots (higher density per unit area), the dark dots are introduced.
In this case, when a plurality of dots having different densities per unit area are controlled, large dots or dark dots are formed after the small dots or light dots have occupied up to 100 percent, if the simple multilevel error diffusion method or dither method is used, or when the method disclosed in the Patent Document 2 is employed.
However, such a method allows the ink to be introduced in excess of the permissible level in some cases, with the result that the image quality will be worsened. Thus, this method will set a limit to the type of the recording medium that can be used.
A proposal for solving such problems is proposed in Patent Documents 3 and 4. To be more specific, this is a technique of allowing the large dots and dark dots to be mixed with the small dots and light dots before the coverage ratio of the small dots and light dots reaches 100 percent.
The Patent Document 3 permits the second dots to be mixed before the first dots reaches 100 percent, by reducing the difference between the threshold value 1 for determining formation of the first dot and the threshold value 2 for determining formation of the second dot (>threshold value 1). This procedure reduces the overall amount of ink.
The Patent Document 4 controls the threshold value 1 and threshold value 2, whereby the percentage of the occurrence of the first and second dots is made to conform to the density to be achieved.
The Patent Documents 5 and 6 use recording rate determining means to determine the recording rate for each level of dot with regard to input value when arranging a plurality of dots having different densities per unit area.
Patent Document 1: Unexamined Japanese Patent Application Publication No. 2004-1311
Patent Document 2: Unexamined Japanese Patent Application Publication No. 2003-80682
Patent Document 3: Official Gazette of Japanese Patent No. 3480924
Patent Document 4: Unexamined Japanese Patent Application Publication 2000-6444
Patent Document 5: Official Gazette of Japanese Patent No. 3208777
Patent Document 6: Official Gazette of Japanese Patent No. 3292104
However, in the aforementioned Patent Document 1, a spurious profile occurs when the dot diameter is changed. If an image is formed using the dot having a smaller diameter, banding resulting from irregular dot formation will stands out more conspicuous. Further, this method fails to ensure a sufficient dispersion of large dots.
The Patent Document 2 cannot impose a restriction to the amount of ink, as described above, and hence sets a limit to the type of the recording medium that can be used. Such problems have been left unsolved in this technique.
According to the Patent Document 3, the resultant dot coverage ratio of the first and second dots cannot be determined uniquely and the gradation is reversed in some cases.
According to the Patent Document 4, the dot ratio in a predetermined range can be determined, but when viewed locally, the dispersion of the second dots is very poor and contrast irregularities occurs easily.
To be more specific, in the technique disclosed in the Patent Documents 3 and 4, large dots are produced in terms of probability, and differences in densities of the dark dots appears easily, with the result that visual irregularities are often found. Such problems have been left unsolved in this technique.
According to the Patent Documents 5 and 6, it is necessary to determine and to memorize the recording rate for each of the plurality of dots having different densities per unit area, and this leads to the problem that the processing for these processes becomes complicate and need long processing time. Further, with regard to the problems of ink amount and banding, it is effective to change the recording rate corresponding to recording method or kind of recording media, therefore, according to the Patent Documents 5 and 6, it is also necessary to determine a recording rate table according to each recording method and recording medium.